New "i15" implementation of big integers (faster, and constant-time, on ARM Cortex...

[BearSSL] / src / rsa / rsa_pkcs1_sig_unpad.c

diff --git a/src/rsa/rsa_pkcs1_sig_unpad.c b/src/rsa/rsa_pkcs1_sig_unpad.c
new file mode 100644 (file)
index 0000000..d179945
--- /dev/null
+++ b/src/rsa/rsa_pkcs1_sig_unpad.c
@@ -0,0 +1,121 @@
+/*
+ * Copyright (c) 2017 Thomas Pornin <pornin@bolet.org>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining 
+ * a copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be 
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, 
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND 
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
+ * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+ * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+#include "inner.h"
+
+/* see bearssl_rsa.h */
+uint32_t
+br_rsa_pkcs1_sig_unpad(const unsigned char *sig, size_t sig_len,
+       const unsigned char *hash_oid, size_t hash_len,
+       unsigned char *hash_out)
+{
+       static const unsigned char pad1[] = {
+               0x00, 0x01, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF
+       };
+
+       unsigned char pad2[43];
+       size_t u, x2, x3, pad_len, zlen;
+
+       if (sig_len < 11) {
+               return 0;
+       }
+
+       /*
+        * Expected format:
+        *  00 01 FF ... FF 00 30 x1 30 x2 06 x3 OID [ 05 00 ] 04 x4 HASH
+        *
+        * with the following rules:
+        *
+        *  -- Total length is that of the modulus and the signature
+        *     (this was already verified by br_rsa_i31_public()).
+        *
+        *  -- There are at least eight bytes of value 0xFF.
+        *
+        *  -- x4 is equal to the hash length (hash_len).
+        *
+        *  -- x3 is equal to the encoded OID value length (so x3 is the
+        *     first byte of hash_oid[]).
+        *
+        *  -- If the "05 00" is present, then x2 == x3 + 4; otherwise,
+        *     x2 == x3 + 2.
+        *
+        *  -- x1 == x2 + x4 + 4.
+        *
+        * So the total length after the last "FF" is either x3 + x4 + 11
+        * (with the "05 00") or x3 + x4 + 9 (without the "05 00").
+        */
+
+       /*
+        * Check the "00 01 FF .. FF 00" with at least eight 0xFF bytes.
+        * The comparaison is valid because we made sure that the signature
+        * is at least 11 bytes long.
+        */
+       if (memcmp(sig, pad1, sizeof pad1) != 0) {
+               return 0;
+       }
+       for (u = sizeof pad1; u < sig_len; u ++) {
+               if (sig[u] != 0xFF) {
+                       break;
+               }
+       }
+
+       /*
+        * Remaining length is sig_len - u bytes (including the 00 just
+        * after the last FF). This must be equal to one of the two
+        * possible values (depending on whether the "05 00" sequence is
+        * present or not).
+        */
+       if (hash_oid == NULL) {
+               if (sig_len - u != hash_len + 1 || sig[u] != 0x00) {
+                       return 0;
+               }
+       } else {
+               x3 = hash_oid[0];
+               pad_len = x3 + 9;
+               memset(pad2, 0, pad_len);
+               zlen = sig_len - u - hash_len;
+               if (zlen == pad_len) {
+                       x2 = x3 + 2;
+               } else if (zlen == pad_len + 2) {
+                       x2 = x3 + 4;
+                       pad_len = zlen;
+                       pad2[pad_len - 4] = 0x05;
+               } else {
+                       return 0;
+               }
+               pad2[1] = 0x30;
+               pad2[2] = x2 + hash_len + 4;
+               pad2[3] = 0x30;
+               pad2[4] = x2;
+               pad2[5] = 0x06;
+               memcpy(pad2 + 6, hash_oid, x3 + 1);
+               pad2[pad_len - 2] = 0x04;
+               pad2[pad_len - 1] = hash_len;
+               if (memcmp(pad2, sig + u, pad_len) != 0) {
+                       return 0;
+               }
+       }
+       memcpy(hash_out, sig + sig_len - hash_len, hash_len);
+       return 1;
+}